1. Field of the Invention
The present invention generally relates to lighting poles and appurtenances mounted along highways and roadways, and, particularly to a multiple necked-down break-away coupling for supporting such lighting poles and other appurtenances which are frangible and facilitate the disengagement of a lighting pole or the like from the base on which it is mounted to minimize bodily and property damage when an automobile collides with a lighting pole or other highway appurtenance.
2. Description of the Prior Art
Many highway and roadside appurtenances, such as lighting poles, signs, etc., are mounted along highways and roads. Typically, these are mounted on and supported by concrete foundations, bases or footings. However, while it is important to securely mount such roadside appurtenances to withstand weight, wind, snow and other types of service loads, they do create a hazard for vehicular traffic. When a vehicle collides with such a light pole or sign post, for example, a substantial amount of energy is normally absorbed by the light pole or post as well as by the impacting vehicle unless the pole or post it is mounted to be readily severed from the base. Unless the post is deflected or severed from the base, therefore, the vehicle may be brought to a sudden stop with potentially fatal or substantial injury to the passengers. For this reason, highway authorities almost universally specify that light poles and the like must be mounted in such a way that they can be severed from the support structure upon impact by a vehicle.
In designs of such break-away couplings several facts or considerations come into play. The couplings must have maximum tensile strength with predetermined (controlled) resistance to bending. Additionally, the couplings must be easy and inexpensive to install and maintain. They must, of course, be totally reliable.
Numerous break-away systems have been proposed for reducing damage to a vehicle and its occupants upon impact. For example, load concentrated break-away couplings are disclosed in U.S. Pat. Nos. 3,637,244, 3,951,556 and 3,967,906 in which load concentrating elements eccentric to the axis of the fasteners, for attaching the couplings to the system oppose the bending of the couplings under normal loads while presenting less resistance to bending of the coupling under impact or other forces applied near the base of the post. In U.S. Pat. Nos. 3,570,376 and 3,606,222, structures are disclosed which include a series of frangible areas. In both cases, the frangible areas are provided about substantially cylindrical structures. Accordingly, while the supports may break along the frangible lines, they do not minimize forces for bending of the posts and, therefore, generally require higher bending energies, to the possible determent of the motor vehicle.
In U.S. Pat. No. 3,755,977, a frangible lighting pole is disclosed which is in a form of a frangible coupler provided with a pair of annular shoulders that are axially spaced from each other. In a sense, the annular shoulders are in the form of internal grooves. A tubular section is provided which is designed to break in response to a lateral impact force of an automobile. The circumferential grooves are provided along a surface of a cylindrical member.
A coupling for a break-away pole is described in U.S. Pat. No. 3,837,752 which seeks to reduce maximum resistance of a coupler to bending fracture by introducing circumferential grooves on the exterior surface of the coupler. The distance from the groove to the coupler extremity is described as being approximately equal to or slightly less than the inserted length of a bolt or a stud that is introduced into the coupler to secure the coupler, at the upper ends, to a base plate that supports the post and to the foundation base or footing on which the post is mounted. The grooves are provided to serve as a stress concentrators for inducing bending fracture and to permit maximum effective length of moment arm and, therefore, maximum bending movement. The invention design is intended to have the grooves provide better control on the bending strength applied by a vehicle before severance or fracture takes place. The grooves in the patent are generally shallow and of a rectangular cross-section. Because the coupler is provided with a central axial bore for the introduction of a bolt, the coupler is cylindrical in form and is not a solid member. Also, because the coupler described in the patent has a cylindrical bore in the middle, very little of the metal is situated on the neutral axis (along the central axis) about which bending takes place, so that, for a given cross-section of material required to withstand a predetermined amount of tensile and compressive stress, the coupler disclosed in the patent is more resistant to bending than a comparable solid member. Because the patent requires that the bolts or studs penetrate at least as deep as the end of the notch or groove, the design is not practical since such design requires that the bolts or studs to bend simultaneously with the coupler about the notch or groove, at least to some degree, during impact. However, it is a major drawback to have the bolts or studs have any effect on the breaking strength of the part since control over the breaking characteristics of the coupler are lost and the point at which the coupler breaks is a function of a system consisting of the coupling as well as the bolt or stud. This patent also requires that the geometry of the shape of the base (the inner or bottom surface) of the groove is to be used to produce the required weakness in bending. According to the patent, the diameter of the neck is not the variable to manipulate in order to achieve the desired strength of the part, as the axial (tensile/compressive) strength is also affected.